Three-dimensional solar cell having increased efficiency

ABSTRACT

A nano-scale tower structure array having increased surface area on each tower for gathering incident light is provided for use in three-dimensional solar cells. Embodiments enhance surface roughness of each tower structure to increase the surface area available for light gathering. Enhanced roughness can be provided by manipulating passivation layer etching parameters used during a formation process of the nano-scale tower structures, in order to affect surface roughness of a photoresist layer used for the etch. Manipulable etching parameters can include power, gas pressure, and etching compound chemistry.

BACKGROUND

1. Field

This disclosure relates generally to photovoltaic systems, and morespecifically, to improving light gathering efficiency ofthree-dimensional solar cells.

2. Related Art

Conventional solar cells present a flat surface to incident light. Onedrawback of flat solar cells is that a significant portion of theincident light is reflected, which reduces the amount of light energyabsorbed by the solar cell. Further, the photovoltaic coating of a flatsolar cell must be thick enough to capture photons of the incidentlight. The energy of the photons liberates electrons from thephotovoltaic materials to create an electrical current with each mobileelectron leaving behind a “hole” in the atomic matrix of thephotovoltaic coating. The longer it takes for electrons to exit thephotovoltaic material (i.e., to flow in a conductive material), the morelikely it is for the electron to recombine with a hole. This reduceselectrical current generated by the solar cell.

Three-dimensional solar cells provide some solutions to the abovedrawbacks of conventional solar cells. Rather than presenting a flatsurface to incident light, a three-dimensional solar cell presents abrush-like surface of nano-scale tower structures. These towerstructures can trap and absorb light received from many differentangles, thereby remaining efficient even when incident light is arrivingat a significant angle to the plane of the solar cell. In addition, thephotovoltaic coatings of the tower structures can be made thinner, whichreduces a likelihood that electron-hole recombination can take place.

One reason that three-dimensional solar cells are more efficient thanflat solar cells at collecting energy from incident light is that thethree-dimensional solar cells present more surface area to capture theincident light then do the flat solar cells. If the light gatheringsurface area of the nano-scale tower structures is increased, then thelight gathering efficiency of three-dimensional solar cells can also beincreased.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerousobjects, features, and advantages made apparent to those skilled in theart by referencing the accompanying drawings.

FIG. 1 is a simplified block diagram illustrating a structure of atypical three-dimensional solar cell.

FIG. 2 is a simplified block diagram illustrating one embodiment of athree-dimensional solar cell having increased area on the towerstructures for collection of incident photonic energy.

FIG. 3 is a simplified block diagram illustrating a cross sectional viewof a three-dimensional solar cell assembly structure at a stage ofprocessing, according to an embodiment of the present invention.

FIG. 4 is a simplified block diagram illustrating the cross sectionalview of the three-dimensional solar cell assembly structure at a laterstage in processing, according to an embodiment of the presentinvention.

FIG. 5 is a simplified block diagram illustrating the cross sectionalview of the three-dimensional solar cell assembly structure at a laterstage in processing, according to an embodiment of the presentinvention.

FIG. 6 is a simplified block diagram illustrating the cross sectionalview of the three-dimensional solar cell assembly structure at a laterstage in processing, according to an embodiment of the presentinvention.

The use of the same reference symbols in different drawings indicatesidentical items unless otherwise noted. The Figures are not necessarilydrawn to scale.

DETAILED DESCRIPTION

A nano-scale tower structure array having increased surface area on eachtower for gathering incident light is provided for use inthree-dimensional solar cells. Embodiments of the present inventionenhance surface roughness of each tower structure to increase thesurface area available for light gathering. In one embodiment, thisenhanced roughness is provided by manipulating photoresist etchingparameters used during a formation process of the nano-scale towerstructures. Manipulable etching parameters can include power, gaspressure, and etching compound chemistry.

A typical three-dimensional solar cell presents a brush-like array ofnano-scale tower structures to capture incident light. A thin coating ofphotovoltaic material is provided on each of the tower structures tocapture and convert the energy from the incident light. Releasedelectrons then flow in a circuit provided, in part, by the conductivecore of the tower structure and a thin conductive coating over thephotovoltaic material.

FIG. 1 is a simplified block diagram illustrating a structure of atypical three-dimensional solar cell. Three-dimensional solar cell 100is formed on a substrate 110, which can be a variety of materialsincluding, for example, a semiconductor, glass, insulator, and the like.A semiconductor substrate can be any semiconductor material orcombinations of materials, such as gallium arsenide, silicon germanium,silicon-on-insulator (SOI), silicon, monocrystalline silicon, and thelike and combinations of the above. An insulating layer 120 is formed onsubstrate 110. In one embodiment, insulating layer 120 is a dielectric(e.g., silicon dioxide, silicon nitride, silicon oxynitride, or anycombination of such layers) formed by an oxidation process or deposited,depending on the nature of the substrate. Choice of materials for thesubstrate and insulating layer can be made depending upon the needs ofthe application.

A conductive layer 130 is provided above insulating layer 120.Conductive layer 130 provides part of the circuit for flow of electronsreleased by incident light in the three-dimensional solar cell.Conductive layer 130 can be any conductive material including, but notlimited to, metal, metal alloy, doped polysilicon, doped amorphoussilicon, nitride or silicide. In one embodiment, conductive layer 130 isa metal alloy including one or more of titanium, aluminum, copper,nickel, and tungsten.

On conductive layer 130 is formed an array of nano-scale towerstructures 140. In three-dimensional solar cells known in the art, thecore material of tower structures 140 can be a variety of conductingmaterials, such as, for example, carbon nanotubes, nickel, zinc, andother metals suitable for forming nano-scale structures. Also formed onconductive layer 130 is a passivation layer 150. As with conventionalsolar cells, passivation layer 150 is provided to reduce surfacerecombination of electrons and holes. Passivation layer 150 can beformed of a variety of insulative oxides and nitride, including, forexample, silicon nitride, silicon oxide, titanium oxide, and the like.

A photovoltaic coating 160 is formed over the surface of nano-scaletower structures 140. Photovoltaic coating 160 provides for conversionof incident photon energy to electrical energy. One example of aphotovoltaic coating appropriate for use in three-dimensional solarcells is cadmium telluride, which provides a p-type photovoltaic layer.Another example of an appropriate photovoltaic coating is cadmiumsulfide, which provides an n-type photovoltaic layer. One method forforming the photovoltaic coating uses molecular beam epitaxy to grow thecoating. Finally, a thin coating of a clear conducting material (notshown) is provided over the photovoltaic layer to serve as the solarcell's top electrode. An example of such a clear conducting material isindium tin oxide.

A typical three-dimensional solar cell will have tower structuresapproximately 100 microns in height and approximately 40 microns inwidth. The towers can be spaced approximately 10 to 20 microns apart inan array.

As shown in FIG. 1, incident light 170 can impact a surface of one ofthe tower structures and have some of the incident light reflect intothe depths of the array of tower structures. The reflected lightprovides photonic energy to the surface of each incident tower structurefor conversion to released electrons. This reflection and subsequentenergy conversion does not happen with a flat solar cell, sincenon-absorbed light is reflected away from the surface of the solar celland lost to energy conversion. This loss of reflected photons isenhanced at large angles of incidence of light. It should be understoodthat FIG. 1 provides a cross-sectional illustration of athree-dimensional array of tower structures, and that the array ofstructures are repeated both above and below the plane of theillustration.

A drawback of traditional three-dimensional solar cells is that althoughthey provide enhanced energy generation from incident light over a flatsolar cell, ultimately incident light reaches passivation layer 150 andis reflected back out of the array of tower structures. To avoid this,embodiments of the present invention provide for additional surface areaon the tower structures to capture incident light. This additionalsurface area is created by increasing surface roughness of the towerstructures. Further, light reflected from the roughened surfaces of thetower structures is scattered, thereby increasing a number of reflected“hits” on surfaces of other tower structures and increasing chances ofabsorption. Thus, the conversion efficiency of the three-dimensionalsolar cell is increased for each unit of incident photonic energy over atraditional three-dimensional solar cell.

FIG. 2 is a simplified block diagram illustrating one embodiment of athree-dimensional solar cell having increased area on the towerstructures for collection of incident photonic energy. Substrate 210,insulating layer 220, conductive layer 230, and passivation layer 250each correspond to layers 110, 120, 130, and 150, respectively.Materials used for the formation of the layers in FIG. 2 can be chosenfrom the same types of materials used for the corresponding layers inFIG. 1.

On conductive layer 230 is formed an array of nano-scale towerstructures 240, similar to the process described above for FIG. 1. Thecore material of tower structures 240 can be a variety of conductingmaterials, such as, for example, nickel, zinc, and other metals suitablefor forming nano-scale structures. Unlike in FIG. 1, however, towerstructures 240 have a surface roughness imparted by processingtechniques described more fully below. The degree of surface roughnessis selectively imparted upon the tower structures in order to enhancethe light-gathering capacity of the three-dimensional solar cell by: (a)increasing the available surface area of the tower structures to gatherincident light energy, and (b) causing enhanced scattering of incidentphotons within the brush-like array of tower structures to increaselikelihood of absorption of light energy on the surface of the towerstructures. As with FIG. 1, the tower structures are provided with aphotovoltaic coating 260 (e.g., CdTe or CdS) to enable the conversion ofincident photons to mobile electrons and a clear conducting material(not shown) to aid in conduction of the electrons to form a circuit. Theroughness of the tower structure sidewalls is repeated in the surface ofthe photovoltaic coating and clear conducting material, as these layersfollow the underlying surface of the tower structures.

FIG. 2 illustrates a reflection path that incident light 270 can followin an array of rough-surfaced tower structures. As can be seen, not onlydoes the surface roughness of the tower structures increase the surfacearea of each tower structure, but also the light can be absorbed andreflected additional times between the towers due to increased lightscattering from the rough surface. The likelihood of energy conversionof a photon to a free electron is thereby increased and the likelihoodof light arriving at the surface of passivation layer 250 and beingreflected away from the three-dimensional solar cell is decreased.

FIG. 3 is a simplified block diagram illustrating a cross sectional viewof a three-dimensional solar cell structure at a stage of processing,according to an embodiment of the present invention. As described above,a substrate 210 is provided over which an insulating layer or adhesionlayer 220 is formed. Conductive layer 230 is formed over adhesion layer220, and passivation layer 250 is formed over conductive layer 230.Choices of materials for layers 210, 220, 230, and 250 are describedabove. Formation of these layers can be performed using techniques knownin the art of semiconductor manufacturing.

A photoresist layer 310 is formed over passivation layer 250. A varietyof photoresist materials known in the art can be used, including, forexample, PMMA, PMGI, phenol formaldehyde resin, and epoxy-basedphotoresists. Photoresist layer 310 can be applied in a manneracceptable for the type of photoresist, as known in the art. In oneembodiment, a photoresist layer 310 is applied as a liquid andspin-coated to ensure a uniform thickness over a panel including theunderlying layers.

FIG. 4 is a simplified block diagram illustrating the cross-sectionalview of the three-dimensional solar cell structure at a later stage inprocessing. Holes 410 are formed in photoresist layer 310 by aphotolithographic process (e.g., patterning and developing). Patterningof the photoresist is performed to lay out regions for holes 410 inaccord with the desired array of tower structures for the ultimatethree-dimensional solar cell. The holes extend through the photoresistlayer to the surface of passivation layer 250.

FIG. 5 is a simplified block diagram illustrating the cross-sectionalview of the three-dimensional solar cell structure at a later stage inprocessing. Holes 510 are formed in passivation layer 250 by a dry etchprocess. As illustrated, holes 510 are formed in a manner that providesrough sidewalls of the holes as they extend through photoresist layer310 to the passivation layer. Surface roughness, or texture, of theholes is selectively determined by manipulating chemistry, pressure,temperature, and energy of the etching process used to form the holes inthe passivation layer. For example, one or more of increasing etchpower, decreasing etch pressure, and increasing flow rate of certainetch chemicals results in a more aggressive etch that results in rougherhole sidewalls than would a less aggressive etch. Table 1 provides oneexample of changes in passivation layer etch process parameters thatdifferentiate between smooth holes in the photoresist and rough holes inthe photoresist. It should be noted that different photoresists anddifferent etch chemistries may have different values and delta values toresult in desired surface roughnesses.

TABLE 1 Smooth Rough Power 1100 W 1300 W Pressure 700 mT 500 mT EtchChemical Flow 16 sccm 32 sccm

Surface roughness is valued in a variety of ways. In one valuation ofsurface roughness, R_(a), a mean of absolute values of profile heights,is measured from a mean line averaged over the profile. For some metals,a surface roughness of 1.6 μm results in sufficient scattering ofincident light that the surface is nearly non-reflective. Embodiments ofthe present invention provide a surface roughness of the hole sidewallsof between 0.1 μm and 1.6 μm such that a surface of the towerssubsequently formed (as discussed below) scatters light to a point ofbeing nearly non-reflective of incident light back out of the towerarray.

FIG. 6 is a simplified block diagram illustrating the cross sectionalview of the three-dimensional solar cell assembly structure at a laterstage in processing. Tower structures 240 are formed by filling holes510 with a metal or metal alloy appropriate to forming nano-structures.In one embodiment, nickel is used to form tower structures 240 and isapplied through an electroplating process. In other embodiments, thetower forming metal is provided to the holes using one of an electrolessplating method, chemical vapor deposition techniques, and physical vapordeposition techniques (e.g., evaporation, sputtering, and the like). Thesidewalls of the metal tower structures assume the same roughnessprofile provided to holes 510, since the holes are a mold for the metaltower structures. For the deposition techniques that form blanket films(e.g., chemical and physical vapor deposition), CMP or an etch step canbe used to isolate the tower structure metal. After completing theprocess for forming the metal tower structures, the remainingphotoresist is removed using, for example, a chemical removal processknown in the art.

After removal of photoresist layer 310, tower structures 240 are coatedwith a photovoltaic layer 260, as illustrated in FIG. 2. As discussedabove, the surface of photovoltaic layer 260 assumes a similar roughnessprofile to that of tower structures 240. Also as discussed above, aclear conducting material such as indium tin oxide is applied to thestructure to provide a conductive path for electrons released byincident photons.

In this manner, improved efficiency of three-dimensional solar cells canbe realized by intentionally applying surface roughness to nano-scaletower structures of such a solar cell. The increased roughness increasesavailable surface area for incident photons to interact withphotovoltaic material. Further, the increased roughness increaseslikelihood of absorption of all photonic energy incident upon thethree-dimensional solar cell by increasing photon reflection among towerstructures of the three-dimensional solar cell.

By now it should be appreciated that a method for forming athree-dimensional photovoltaic cell has been provided, in which themethod includes forming a conductive layer over a substrate, forming apassivation layer over the conductive layer, forming a photoresist layerover the passivation layer, patterning the photoresist layer for one ormore holes, etching the passivation layer to form the one or more holessuch that a sidewall of each of the one or more holes through thephotoresist layer has a surface roughness, forming a conductive materialin a hole of the one or more holes, and removing the photoresist layer.The sidewalls of the conductive material have the surface roughness fromthe sidewall of the corresponding hole through the photoresist layer.Etching the passivation layer is performed such that the sidewallsurface roughness of the conductive material will provide a selectedlevel of scattering of light incident on the conductive materialsidewall.

In one aspect of the above embodiment forming the conductive materialincludes an electroplating process. In a further aspect, the conductivematerial includes one of a metal or a metal alloy. In yet a furtheraspect, the conductive material can include one of nickel ore zinc.

In another aspect of the above embodiment, increasing the surfaceroughness of the hole sidewalls through the photoresist layer isperformed by increasing power used during the etching process. In adifferent aspect of the above embodiment, increasing the surfaceroughness of the hole sidewalls through the photoresist layer isperformed by decreasing the pressure of gas is used during the etchingprocess. In still a different aspect of the above embodiment, increasingthe surface roughness of the hole sidewalls through the photoresistlayer is performed by increasing a flow rate of one or more gases usedduring the etching process. And in another aspect of the aboveembodiment, the etching is performed to selectively provide a surfaceroughness of the hole sidewalls through the photoresist layer of between0.1 μm and 1.6 μm.

Another embodiment provides for a three dimensional photovoltaic devicethat includes a plurality of tower structures having a photovoltaiccoating along a top and sidewalls of each tower structure, a conductivelayer formed over a substrate, and a passivation layer formed over theconductive layer. The sidewalls of each tower structure have a surfaceroughness providing a selected level of scattering of light incident onthe tower structures and each tower structure has a conductive corematerial. The conductive core material of each tower structure iscoupled to the conductive layer.

In one aspect of the above embodiment, the plurality of tower structuresare performed by a process including: forming the passivation layer overthe conductive layer, forming a photoresist layer over the passivationlayer, patterning the photoresist layer for a plurality of holes,etching the passivation layer to form the plurality of holes, formingthe conductive core material in the one or more holes through both thepassivation layer and photoresist layer, and removing the photoresistlayer. In this process, the etching is performed such that a sidewall ofeach of the plurality of holes through the photoresist layer has asurface roughness. In a further aspect, the surface roughness of theholes provides the selected level of scattering of light incident on thetower structures.

In another aspect of the above embodiment, a tower structure of theplurality of tower structures is approximately 10 microns in height overthe conductive layer and approximately 4 microns in diameter. In afurther aspect, the surface roughness of the tower structures is between0.1 and 1.6 microns.

In yet another aspect of the above embodiment, the conductive corematerial includes one of a metal or a metal alloy. In a further aspect,the conductive core material includes nickel or zinc.

In another aspect of the above embodiment, the photovoltaic coating isone of cadmium telluride or cadmium sulfate.

Although the invention has been described with respect to specificconductivity types or polarity of potentials, skilled artisansappreciated that conductivity types and polarities of potentials may bereversed.

Moreover, the terms “front,” “back,” “top,” “bottom,” “over,” “under”and the like in the description and in the claims, if any, are used fordescriptive purposes and not necessarily for describing permanentrelative positions. It is understood that the terms so used areinterchangeable under appropriate circumstances such that theembodiments of the invention described herein are, for example, capableof operation in other orientations than those illustrated or otherwisedescribed herein.

Although the invention is described herein with reference to specificembodiments, various modifications and changes can be made withoutdeparting from the scope of the present invention as set forth in theclaims below. For example, p-type semiconductor materials can bereplaced with n-type semiconductor materials. Accordingly, thespecification and Figures are to be regarded in an illustrative ratherthan a restrictive sense, and all such modifications are intended to beincluded within the scope of the present invention. Any benefits,advantages, or solutions to problems that are described herein withregard to specific embodiments are not intended to be construed as acritical, required, or essential feature or element of any or all theclaims.

Furthermore, the terms “a” or “an,” as used herein, are defined as oneor more than one. Also, the use of introductory phrases such as “atleast one” and “one or more” in the claims should not be construed toimply that the introduction of another claim element by the indefinitearticles “a” or “an” limits any particular claim containing suchintroduced claim element to inventions containing only one such element,even when the same claim includes the introductory phrases “one or more”or “at least one” and indefinite articles such as “a” or “an.” The sameholds true for the use of definite articles.

Unless stated otherwise, terms such as “first” and “second” are used toarbitrarily distinguish between the elements such terms describe. Thus,these terms are not necessarily intended to indicate temporal or otherprioritization of such elements.

What is claimed is: 1-8. (canceled)
 9. A three-dimensional photovoltaic device comprising: a plurality of tower structures having a photovoltaic coating along a top and sidewalls of each tower structure, wherein the sidewalls of each tower structure have a surface roughness providing a selected level of scattering of light incident on the tower structures, and each tower structure has a conductive core material; a conductive layer formed over a substrate, wherein the conductive core material of each tower structure is coupled to the conductive layer; and a passivation layer formed over the conductive layer.
 10. The three-dimensional photovoltaic device of claim 9, wherein the plurality of tower structures are formed by a process comprising: forming the passivation layer over the conductive layer; forming a photoresist layer over the passivation layer; patterning the photoresist layer for a plurality of holes; etching the passivation layer to form the plurality of holes, wherein said etching is performed such that a sidewall of each of the plurality of holes through the photoresist layer has a surface roughness; forming the conductive core material in the one or more holes through both the passivation layer and the photoresist layer, and removing the photoresist layer.
 11. The three-dimensional photovoltaic device of claim 10 wherein the surface roughness of the holes provides the selected level of scattering of light incident on the tower structures.
 12. The three-dimensional photovoltaic device of claim 9 wherein a tower structure of the plurality of tower structures is approximately 10 microns in height over the conductive layer and approximately 4 microns in diameter.
 13. The three-dimensional photovoltaic device of claim 12 wherein the surface roughness of the tower structure is between 0.1 and 1.6 microns.
 14. The three-dimensional photovoltaic device of claim 9 wherein the conductive core material comprises one of a metal or a metal alloy.
 15. The three-dimensional photovoltaic device of claim 14 wherein the conductive core material comprises nickel or zinc.
 16. The three-dimensional photovoltaic device of claim 9 further comprising: a passivation layer formed over the conductive layer.
 17. The three-dimensional photovoltaic device of claim 9 wherein the photovoltaic coating is one of CdTe or CdS. 